A number of specifications disclose the treatment of used cooking oil (includes vegetable oils and animal fats) from fat fryers in order to prolong the life of the oil.
Cooking oils are triglycerides whose structure is exemplified by the following compound having two radicals of oleic acid and one radical of palmitic acid attached to glycerol:
and additionally oils having as substituents multiply unsaturated fatty acid radicals e.g. linoleyl:—C(O)(CH2)7CH═CH(CH2)CH═CH(CH2)4CH3 
The following indicates the distribution of fatty acids in some common cooking oils, linolenic being—C(O)(CH2)7CH═CH(CH2)CH═CH(CH2)CH═CH(CH2)CH3
OilLinolenic %Linoleic %Oleic %Saturated %Corn1.6752.6830.5115.15Rapeseed6.7623.5658.3911.29Sunflower0.9560.2926.5712.19Olive, refined1.215.5978.6214.57Soyabean7.9152.5725.5713.95GM Soyabean1.0158.7725.9414.28
Deterioration of oil begins on exposure of oil to air or moisture. Simple storage of oil at ambient temperature can give rise to degradation, see e.g. Giullén et al., Detection of Primary and Secondary Oxidation Products by Fourier Transform Infrared Spectroscopy (FTIR) and 1H Nuclear Magnetic Resonance (NMR) in Sunflower Oil during Storage, J. Agric. Food Chem. 2007, 55, 10729-10736, 10729, the authors investigating the slow oxidation of edible oils and in particular sunflower oil on oxidation in the presence of air. Sunflower oil is a material of particular interest since it is used by most commercial food fryers.
Heating of oil during deep drying of food in oils whether the oil is in a hot stand-by mode or whether frying is being carried out gives rise to degradation products that contaminate the oil and have undesirable effects.
Hydrolysis by moisture or by the steam of cooking gives rise to free fatty acid which has surfactant properties and reduces the surface tension of the oil. As a result batter and breading absorb additional oil, giving rise to greasy fried food, and additionally the smoke point of the oil is reduced.
Oxidative degeneration of oils or fatty acids contained therein is free radical initiated and leads to various decomposition products including organic peroxides, alcohols, aldehydes, ketones, carboxylic acids, and high molecular weight materials. The oxidation process begins with the contact of air with hot oil or fatty acid therein and the ultimate creation of oxidized fatty acid (OFA). Continued heating transforms the OFA into secondary and tertiary by-products.
Contaminants in cooking oil are becoming of increasing concern from a health standpoint.
For example, Grootveld et al., Food Chemistry, 67 (1999) 211-213 warns that the formation of cytotoxic aldehydes in cooling oil during routine frying could be a health hazard.
Further undesirable contaminants in cooking oil are trans fats whose content in oil in a deep fryer may increase over time, especially if there is used an oil rich in ω-3 fatty acids e.g. canola or rapeseed oil. Scientific evidence shows that consumption of saturated fat, trans fat, and dietary cholesterol raises low-density lipoprotein (LDL), or “bad cholesterol,” levels, which increases the risk of coronary heart disease (CHD). NYC banned cooking oils with trans fats from July 2007 and any trans-fat additives from July 2008. However, tests show that fatty acids including other toxic, mutagenous and carcinogenous chemicals, such as aldehydes, are actually generated when deep fat frying. Even in GM modified soyabean oils where the linolenic content has been reduced in favor of linoleic, trans fats will still form during the cooking process.
Various methods have been proposed for withdrawing cooking oil from a cooker where it is used, subjecting it to one or more purification treatments and returning the treated oil to the cooker. U.S. Pat. No. 3,947,602 (Vlewell et al., Bernard) discloses that the useful life of cooking oil is increased by treating the cooking oil with a food compatible acid and generally also with a suitable adsorbent such as an activated carbon. U.S. Pat. No. 4,112,129 (Duensing et al., Johns Manville) discloses filtering the oil through a composition comprising 47 to 59 parts by weight diatomite (70-80 wt % SiO2), 28 to 36 parts by weight synthetic calcium silicate hydrate, and 12 to 24 parts by weight synthetic magnesium silicate hydrate. U.S. Pat. No. 4,330,564 (Bernhard) discloses a process for treating used fryer cooking oil comprising the steps of mixing said used cooking oil at a temperature of from about 150-200° C. with a composition comprising porous carrier e.g. rhyolite, water and food compatible acid e.g. citric acid and filtering the residue of said composition from said oil. US-A-2005/0223909 Kuratu) discloses filtering the oil through granite porphyry.
The effect of different absorbents on purification of used sunflower seed oil has been reviewed by Maskan et al., Eur Food Res Technol (2003) 217:215-218. The refining of used sunflower seed oil was investigated by various adsorbent treatments. Six adsorbents, CaO, MgO, Mg2CO3, magnesium silicate, activated charcoal and bentonite, as well as an available natural earth (i.e. pekmez earth, CaCO3 containing special natural white soil) were studied. Pekmez earth, magnesium silicate (florisil) and bentonite exhibited the highest abilities in viscosity, free fatty acids (FFAs) reduction and colour recovery, respectively, among the adsorbents studied. Therefore, a mixture of 2% pekmez earth, 3% bentonite and 3% magnesium silicate was found to be the best combination. However the presence of adsorbents during the frying process was not disclosed.
Other methods have been proposed for treating cooking oil in situ in a cooker. U.S. Pat. No. 4,764,384 (Gyann, GyCor International) discloses that spent cooking oil may be rejuvenated by directly adding to the spent cooking oil in the fryer filtering media containing particles of material which become uniformly suspended throughout the liquid body of the spent cooking oil, the particles of filtering media material being effective to absorb contaminants and bleach the spent cooking oil to extend its useful life. The filtering media comprises synthetic amorphous silica provided with moisture, synthetic amorphous magnesium silicate, and diatomaceous earth. U.S. Pat. No. 4,681,768 (Mulflur et al.) discloses treating used cooking oils and/or cooking fats so as to permit use thereof over longer periods of time by contacting the oil or fat with an activated hydrated synthetic magnesium silicate which has a surface area of at least 300 m2/g. The magnesium silicate functions to adsorb polar compounds of degradation such as FFA, OFA, colour bodies and secondary and tertiary by-products of degradation, which can be subsequently removed during the normal filtration of the used oil and/or fat. The magnesium silicate may function as a filter aid, or may be employed in conjunction with another filter aid during the filtration of the oil, as generally practiced in the art. U.S. Pat. No. 5,354,570 (Friedman, Oil Process Systems) discloses a method of frying food in cooking fluid within which degradation products comprising surfactants are produced therein and food residue accumulates, wherein there is added a treatment compound e.g. a porous rhyolitic material in the form of a powder capable of selectively reducing the amount of said surfactants in said used cooking fluid, and wherein the treatment compound is permitted to remain within said fryer apparatus and to settle upon said food residue while continuing said food frying process. U.S. Pat. No. 5,391,385 (Seybold, P Q Corporation) discloses the hot treatment of oil with a mixture of 60-80% amorphous silica and 20-40% alumina. The mixture can be placed in a permeable container which is then placed in the oil, the container being permeable to the oil but not to the mixture so that the adsorbent is not released into the oil and filtration is not required. When the mixture is spent, the container of the mixture can be removed from the oil. JP-A-07-148073 (Yoshihide) discloses finely pulverized zeolite stones inserted into bag of filter material to form a package which may be put into a cooking vessel together with oil and a cooking material, and cooked together.